1. Field of the Invention
The present invention relates to machine tools such as bending presses and punching machines for sheet metal.
2. Description of the Prior Art
Conventional bending presses intended to bend a sheet metal have a very simple structure in which two or more C-shaped pillars carry a pair of facing plates, the upper or lower one of which is movable vertically towards the other plate. The horizontal facing edges of the plate carry the bending tools constituted by a linear punch and die. In the most common arrangement, the die is carried by the lower plate and the punch is carried by the upper plate.
The movement of the movable plate is achieved by electrical or hydraulic means which move it exactly parallel to the vertical working plane defined by the die and the punch.
In conventional bending presses, the C-shaped pillars constitute a force structure which transmits the mutual force between the punch and the die. In large presses this force may reach many hundreds of tons.
The force structure, however, also has the function of guiding the mutual movement of the die and the punch precisely not only in the vertical working plane but also in vertical planes perpendicular to the working plane in order to prevent as far as possible undesirable deviations of the bending line from the correct position.
Accordingly, in the bending press having the structure mentioned above, during the bending operation, the force structure undergoes elastic deformation, and the precision of the bending operations deteriorates.
Punches are machines which are able to form holes of any size in a metal sheet, as well as trimming it and cutting the outline by means of blanking operations.
The conventional punches include a massive structure, usually in the form of a C but sometimes, for reasons of rigidity, taking the form of a closed ring. At least one die and a corresponding punch are fixed so as to be perfectly coaxial to the lower part and the upper part of the C-shaped structure respectively. In practice, a punch has a multiplicity of dies and punches which are rapidly interchangeable in various ways, for example, by means of rotary turrets.
In the case of a punch with the C-shaped structure, the structure also has a force function and transmits the mutual force between the punch and the die, which is very large punches may reach even tens or sometimes hundreds of tons. A punch differs from a bending machine in that the blanking is carried out at a high speed and the force is typically impulsive.
In addition to the force function, the C-shaped structure or ring structure of the punch must also fulfil a precision function, that is, of maintaining the die and the punch in perfect alignment. Moreover, it must support the numerically-controlled drive members for the metal to ensure the necessary repeatability and precision of the movements.
These precision functions are extremely onerous because of the vibrations caused by the impulsive punching forces.
Further, a horizontal ball table of known type is associated with conventional punching machines for supporting and sliding the metal sheet to be worked. This table is situated in front of the opening of the punching space and is extended by another coplanar ball table within the space itself.
The metal sheet which rests on the ball table is gripped along an edge remote from the punching space by grips or clamps driven by a numerically-controlled movement system along two horizontal axes which are perpendicular to each other.
By virtue of this system, the metal sheet can be moved very rapidly in the working area for the execution of various punching operations at any point on the metal sheet, whilst it rests on the ball table.
In conventional punching machines, the ball table is used to reduce as much as possible the sliding friction of the metal sheet which is moved horizontally by the clamps driven by the numerically-controlled system. However, even ball tables have friction which is not negligible, particularly in the case of large metal sheets; as an indication, it is sufficient to realize that the punching machines which are currently available on the market are able to deal with metal sheets of 2500.times.2500.times.6 mm, which may weigh 175 kg if they are of steel.
Yet another disadvantage of ball tables, which is as serious as friction for the purposes of precision, is that, during punching, the balls constitute localized point supports which may even correspond to regions already punched. The losing and regaining of the contact between the ball and the metal sheet in the punched region causes an uncontrolled disturbance in the accuracy of the positioning of the metal sheet on the table.